Thin magnetic films



United States Patent 3,117,896 'll-ilN P/iA GNETlC FEMS tl'illiam W. L. Chu, Scotia, and Bernard C. Wagner, Troy, N.Y., assigucrs to General Electric Company, a

corporat n of New Yorlt No Drawing. Filed Apr. 1, 1960, Set. No. 19,182

Claims. (Cl. 148l0$) This invention relates to thin magnetic films. More particularly, it relates to thin magnetic films of desirable uniform character which are useful in computing machines and other applications.

The use of thin magnetic filrns in computers has gained widespread attention in the recent past. For such use, it is essential that the magnetic films have an essentially square or rectangular hysteresis loop. It is also desirable that the coercive force and anisotropy field be reduced to a uniform minimum range. Heretofore, thin magnetic ifilm arrays consisting essentially of iron and nickel in proportions ranging from about 70% to 83% nickel and with the remainder iron, the films being evaporated on a suitable substrate in small spaced segments, have been suggested for high speed computers. However, it has been found that such films have generally been deficient in their magnetic characteristics in that uniform results from one film segment to another were not attainable. Among the factors which are believed to influence the magnetic characteristics of such thin films are crystal orientation, internal stresses and strains, oxidation and film shape, among others.

A principal object therefore of this invention is to provide thin magnetic films having desirable magnetic charact ristics and particularly anisotropy fields which are held to a uniform minimum range.

Briefly, the invention relates to thin magnetic films which are most carefully prepared under controlled conditions beginning with the alloy preparation itself. The alloys further are evaporated from an area source which is as large as the finished magnetic fil n array itself under specifically controlled conditions in the presence of a magnetic field after which the film is subjected to annealing in a transverse field.

Those features of the invention which are believed to be novel are set forth with particularity in the claims appended hereto. The invention will, however, be better understood from a consideration of the following description Wn reby other advantages and objects thereof will be realized.

The preparation of the present uniform magnetic films as pointed out above starts with the preparation of the evaporable alloy itself. The alloys are formed by vacuum melting of high purity materials, the ingots being formed into suitable shape by cold rolling with intermediate annealing as necessary. For example, 99.9% pure iron and nickel are melted under a vacuum under the order of 10- mm. Hg in the desired proportions with cooling carried out under the same vacuum conditions. The ingot is then typically rolled to a flat strip in from 2 to 3 stages with intermediate annealing in vacuum at from about 900 C. to 1000" C. for periods of from about to 60 minutes, depending upon the annealing temperature. The final thiclmess of the strips ranges from about 0.01 to 0.005", the strips being finally annealed again at 900 to 1000 C. for from about 20 to 60 minutes. As will become apparent hereinafter, the preferred specific alloy composition for purposes of this invention is 75% nickel and iron for reasons of uniformity. On the other hand, it has been found that alloys ranging from about 70% nickel to 83% nickel with the remainder iron give approximately the same uniform magnetic characteristics if they are prepared according to this invention. As use in the specification and claims to describe the alloy com- Billfidfi Patented data. 14, 1964 positions of the invention, the term percent refers to weight percent of the metal element in the composition.

In evaporating the alloy strips, it is essential that the melting source be of such shape and dimensions as to provide a surface area sufliciently large to cover, that is, equivalent to the substrate surface upon which the magnetic material is to be deposited. This allows for rapid evaporation of the alloy and uniform magnetic characteristics and is in direct contradistinction to so-called point evaporant sources or sources which are lesser in extent than the film supporting substrate and the desired film itself. Such reduced area sources produce poor magnetic qualities, such as non-uniform anisotropy fields. By the present method of evaporating, the evaporation is carried out very rapidly with the evaporant proceeding essentially directly or perpendicularly from the evaporating surface to the film substrate. In order to facilitate rapid heating without harmful reaction with the evaporant alloy, it is preferred to use fiat tungsten strips of suitable size which are heated by electrical resistance. An alternative method is to use a high melting point ceramic crucible as of alumina with the evaporant placed therein heated preferably by high frequency electrical induction. In the case of the crucible, it is, of course, again necessary that the area of the crucible and the evaporant alloy surface therein be equal to that of the film to be formed. With the preferred tungsten strips, the surfaces thereof ar very carefully cleaned with detergent and by heating to an elevated temperature of the order of from about 1000" C. to 1600 C. for about 1 minute in a high vacuum of the order of 10- mm. Hg. If a ceramic crucible is to be used, it is thoroughly baked out at a temperature and vacuum similar to that used for the tungsten heating strips.

The general conditions for evaporating which have been found to contribute to the superior thin magnetic films provided herein include the use of a high vacuum and a rapid evaporation rate which is facilitated by the large area evaporant as well as a warm substrate on which the thin magnetic film is to be deposited. These conditions when properly met held deposited films of fine crystalline structure and reproducible composition. Furthermore, the metal foil evaporant in having a surface equivalent to that of the substrate minimizes undesirable angle of incidence effects.

The preferred substrate surface on which the film is to be deposited is prepared from microscope cover slides which have been found to be very smooth and uniform. Surface contaminants are removed by washing in a 10% hydrofluoric acid solution, washing thoroughly in distilled water and then in acetone and drying in a stream of inert gas, such as nitrogen, and the like. The substrate is masked with a suitable material such as aluminum, stainless steel, etc., except for the segments of an array of squares upon which the metallic film is to be deposited.

In carrying out the actual evaporation process the substrate, using a tungsten heater, is placed in a bell jar or other container arranged for evacuation along with means for heating, the film holding substrate being arranged over the heater strips and the alloy strips to be evaporated placed on the tungsten. When using a crucible, the arrangement is similar. The system is evacuated to the order of about 10 mm. Hg and the substrate is warmed to a temperature of from about 250 C. to 350 C. and preferably to 300 C. The heated substrate contributes to the quality of the product. The alloy on the heater strip is quickly evaporated by rapidly heating it to a temperature of from about 1500 C. to 1900 C. and most preferably to about 1600 C. for a time sufiicient to allow the deposition of the desired film thickness on the substrate. For example, under the above preferred conditions 60 seconds has been found to provide for the deposition of a 3000 A. thick film. After deposition of the film which may range from several hundred A. upward in thickness, the substrate is cooled in the vacuum to a temperature of about 72 C. after which the system may be opened Without risk of excessive film oxidation. Preferably, dry inert gas, such as nitrogen, is bled into the container at about 150 C. and the substrate and film is removed when the temperature has dropped to about 75 C. During the film deposition process, the substrate and the heater strips are held in the plane of a magnetic field having a strength of about oersteds.

After removal from the evaporating apparatus, the films in the form of segmented arrays are annealed in a transverse field to further develop their magnetic characteristics. The preferred temperature of annealing ranges from about 250 C. to about 350 C. at a field strength of from about 30 to 190 oersteds for times ranging from about 30 minutes to 180 minutes. The specific preferred anneal is carried out at 300 C. for 60 minutes in a 90 oersteds transverse field.

Generally, it has been found insofar as the transverse magnetic anneal is concerned that the rate and magnitude of decrease, that is, improvement in the anisotropy field (H is insignificant at temperatures below 250 C. The H value drops rapidly at 300 C. and then remains constant for periods up to at least 3 hours at a level of 2.5 to 3.5 oersteds. At 350 C., the rates of decrease of H are somewhat faster than at 300 'C., the extents of the decrease being similar. With increased annealing time, the H value was raised from the minimum up to about 4 to 6 oersteds.

It has been found that films consisting of 70%-83% nickel, 17%-30% iron and specifically 75% nickel-25% iron prepared according to this invention possess very desirably low and uniform anisotropy fields. Typical anisotropy fields for these materials are about 3 oersteds with variations of only about plus or minus 17%. The switching speeds or magnetization reversal of the present materials are also very high being comparable to the best reported speeds, i.e., in the order of millimicro seconds. Their coercive force, another very important factor in computer design, is also very desirable, being uniformly about 1.5 to 3 oersteds.

There are prepared by the present invention thin magnetic films useful in computing devices, such materials being produced under rigidly controlled conditions to provide materials of desirable crystal isotropy having low internal stresses and strains, low surface oxidation and desirable film shape which contribute to the uniformity of the physical structure of the film itself and thus to the uniformity of its magnetic characteristics.

What We claim as new and desire to secure by Letters Patent of the United States is:

1. The process of preparing thin magnetic films possessed of superior, uniform magnetic characteristics which comprises rapidly evaporating under vacuum an alloy consisting of by weight about 70% to 83% nickel and from about 17% to 30% iron from an evaporant source having the same area as the film to be deposited while under the influence of a magnetic field in the plane of the film being formed, annealing said film in a transverse field at a temperature of from about 250 C. to about 350 C. and at from about 30 to 190 oersteds for times ranging for about 30 minutes to 180 minutes, said film being charterized by rapid switching speeds and a uniform anisotropy field of about 3 oersteds and a coercive force of cfrom about 1.5 to 3 oersteds.

2. The process of preparing thin magnetic films possessed of superior uniform magnetic characteristics which comprises rapidly evaporating under vacuum an alloy consisting of by weight about 75 nickel and 25 iron from an evaporant source having the same area as the film to be deposited while under the influence of a magnetic field in the plane of the film being formed, and annealing said film in a transverse field at a temperature of 300 C. and

V at 90 oersteds for about 60 minutes.

3. The process of preparing thin magnetic films possessed of superior uniform magnetic characteristics which comprises rapidly evaporating under vacuum an alloy consisting of by weight about 70% to 83% nickel and about 17% to 30% iron from an evaporant source, said evaporant source consisting of strips of alloy of the desired composition prepared by rolling vacuu-m-melt-produced ingots to strip form in from two to three stages with intermediate annealing at from about 900 C. to 1000 C. for from about 20 to 60 minutes with a final anneal of t e same temperature and duration, said evaporant source having the same area as the film to be deposited, imposing a magnetic field in the plane of the film during formation, and annealing said film in a transverse magnetic field of from about 30 to 190 oersteds at temperatures of from etaoni shrdlu crnfwyp vbgkq, vbgkqj Xzfifiiffii xzfifliffii X about 250 C. to 350 C. for from about 30 minutes to 180 minutes.

4. The process of preparing a thin magnetic film possessed of superior uniform magnetic characteristics which comprises rapidly evaporating under vacuum an alloy consisting of by weight about nickel and about 25 iron from an evaporant source, said evnporant source consisting of a strip of alloy of the desired composition prepared by rolling vacuum-melt-produced ingots to strip form in from two to three stages with intermediate annealing at from about 900 C. to l000 C. for from about 20 to 60 minutes with a final anneal of the same temperature and duration, said evaporant source having the same area as the film to be deposited, imposing a magnetic field in the plane of the film during formation and annealing said film in a transverse magnetic field of from about 30 to 190 oersteds at temperatures of from about 250 C. to 350 C. for from about 30 min utes to minutes.

5. The process of preparing a thin magnetic film possessed of low, uniform magnetic anisotropy field, low coercive force and rapid magnetization reversal speed which comprises rapidly evaporating under vacuum an alloy containing from about 70% to 83% nickel and from about 15% to 30% iron mounted on a tungsten strip, held at a temperature of from about 1500 C. to 1700 C., the evaporant source having the same area as the film to be deposited, the substrate for reception of the film being held at a temperature of from about 250 C. to 350 C., under the influence of a magnetic field having a strength of about 10 oersteds directed in the plane of the film being formed, and annealing said film in a transverse magnetic field at a temperature of from about 250 C. to about 350 C. at a field strength of from about 30 to oersteds for from about 30 minutes to 180 minutes.

6. The process of preparing thin magnetic film arrays, comprising spaced segments of film, possessed of superior uniform magnetic characteristics which comprises rapidly evaporating under vacuum an alloy consisting of by weight about 70% to about 83% nickel and from about 17% to 30% iron from an evaporant source having the same area as the film array to be deposited, the substrate upon which the film is to be deposited being held at a temperature from about 250 C. to 350 C. and masked for the reception of spaced film deposits of desired segmental area, said substrate being held under the influence of a magnetic field directed in the plane of the film being formed and annealing said film in a transverse magnetic field of from about 30 to 190 oersteds for about 30 minutes to 180 minutes at a temperature of from 250 C. to about 350 C.

7. The process of preparing thin magnetic film arrays possessed of superior uniform magnetic characteristics which comprises rapidly evaporating under vacuum an alloy consisting of by weight about 75 nickel to about 25% iron from an evaporant source having the same area as the film array to be deposited, the substrate upon which the film is to be deposited being held at a temperature from about 250 C. to 350 C. and masked for the reception of film deposits of desired segmental area, said substrate being held under the influence of a magnetic field directed in the plane of the film being formed and annealing said film in a transverse field of from about 30 to 190 oersteds for about 30 minutes to 180 minutes at a temperature of from 250 C. to about 350 C.

8. A thin magnetic film consisting of by Weight about 70% to 83% nickel and 17% to 30% iron characterized by superior uniform magnetic characteristics including an anisotropy field of about 3 oersteds plus or minus 17%, a coercive force of from about 1.5 to 3 oersteds and magnetization reversal speeds of the order of millimicro seconds.

9. A thin magnetic film consisting of by Weight about 75% nickel and 25% iron characterized by superior uniform magnetic characteristics including an anisotropy field of about 3 oersteds plus or minus 17%, a coercive force of from about 1.5 to 3 oersteds and magnetization reversal speeds of the order of millirnicro seconds.

10. A thin magnetic film array consisting of spaced segments of thin magnetic films possessed of superior uniform magnetic characteristics including an anisotropy field of about 3 oersteds plus or minus 17%, a coercive force of from about 1.53 oersteds and magnetization reversal speeds of the order of millimicro seconds, the magnetic film consisting of by weight about 70-83% nickel and 1730% iron.

References Cited in the file of this patent UNITED STATES PATENTS 2,853,402 Blois Sept. 23, 1958 2,999,766 Ashworth et al. Sept. 12, 1961 3,039,871 Mitchell June 19, 1962 OTHER REFERENCES 

1. THE PROCESS OF PREPARING THIN MAGNETIC FILMS POSSESSED OF SUPERIOR, UNIFORM MAGNETIC CHARACTERISTICS WHICH COMPRISES RAPIDLY EVAPORATING UNDER VACUUM AN ALLOY CONSISTING OF BY WEIGHT ABOUT 70% TO 83% NICKEL AND FROM ABOUT 17% TO 30% IRON FROM AN EVAPORANT SOURCE HAVING THE SAME AREA AS THE FILM TO BE DEPOSITED WHILE UNDER THE INFLUENCE OF A MAGNETIC FIELD IN THE PLANE OF THE FILM BEING FORMED, ANNEALING SAID FILM IN A TRANSVERSE FIELD AT A TEMPERATURE OF FROM ABOUT 250*C. TO ABOUT 350*C. AND AT FROM ABOUT 30 TO 190 OERSTEDS FOR TIMES RANGING FOR ABOUT 30 MINUTES TO 180 MINUTES, SAID FILM BEING CHARTERIZED BY RAPID SWITCHING SPEEDS AND A UNIFORM ANISOTROPY FIELD OF ABOUT 3 OERSTEDS AND A COERCIVE FORCE OF FROM ABOUT 1.5 TO 3 OERSTEDS. 